Intraluminal prostheses used to maintain, open, or dilate blood vessels are commonly known as stents. Stents have been developed for use in various lumens of the body, including, for example, the biliary tree, venous system, peripheral arteries, and coronary arteries. Stent constructions generally include cylindrical frames that define a plurality of openings.
There are two broad classes of stents: self-expanding stents and balloon expandable stents. Self-expanding stents are typically characterized by intraluminal expansion when a constraining force is removed, such as an outer sheath of a stent delivery system, anchor in the presence of an elevated temperature (due to material properties thereof). Self-expanding stents are generally loaded into a stent delivery system by collapsing the stent from an expanded configuration at a first larger diameter to a collapsed configuration at a second smaller diameter. Balloon expandable stents are typically characterized by intraluminal expansion via an inflation force, such as a balloon catheter. Balloon expandable stents are generally loaded onto a balloon catheter through a crimping process to transition the stent to a collapsed configuration, and are plastically deformed when the balloon is inflated in the body vessel to the expanded configuration.
There are two basic architectures for stents, circumferential and helical. Circumferential configurations generally include a series of cylindrical rings, formed by a series of connected struts, joined together by connecting elements or bridges along a longitudinal axis of the stent. Helical configurations include a continuous helical structure along the longitudinal axis of the stent with adjacent windings, formed by a series of connected struts, connected by one or more connecting elements or bridges.
Stents for use in the arterial and venous systems can be made by machining a pattern of struts and connecting elements from a metal tube, typically by laser machining the pattern into the tube. The pattern of struts and connecting elements can be configured depending on the desired attributes. For example, the pattern can be configured to enhance flexibility or bendability. The pattern can also be configured to ensure uniform expansion and prevent foreshortening of the stent upon intraluminal expansion.
Synthetic vascular grafts are routinely used to restore the blood flow in patients suffering from vascular diseases. For example, prosthetic grafts made from expanded polytetrafluoroethylene (ePTFE) are commonly used and have shown favorable patency rates, meaning that depending on a given time period, the graft maintains an open lumen for the flow of blood therethrough. Grafts formed of ePTFE include a microstructure characterized by spaced apart nodes connected by fibrils, the distance between the nodes defined as internodal distance (IND), and are generally extruded either as a tube or as a sheet or film that is fashioned into a tube. Grafts can also be created from fibers woven or knitted into a generally tubular shape. Stents may be fully or partially covered with a graft material, such as ePTFE, on the stent's luminal surface, abluminal surface or both luminal and abluminal surfaces.
Stents may include image enhancing features so that they can be viewed fluoroscopically following intraluminal deployment. Examples of such features include radiopaque markers attached to the stent or integral with the stent, or attached to the one or more graft layers associated with the stent. The image enhancing features generally include a material that is highly visible under fluoroscopy, such as gold, platinum, tantalum, and alloys thereof.